1. Field of the Invention
The invention in general relates to multibeam radars and more particularly to an active electronically scanned array multibeam radar which is particularly well adapted to obtain high resolution radar images.
2. Description of Related Art
In an active electronically scanned array multibeam radar, with a single aperture, i.e. an antenna, various modes of operation may be accomplished including a synthetic aperture radar (SAR) mode as well as a surface or ground moving target indicator (MTI) mode. The antenna is comprised of a multitude of individual antenna elements each of which has its own phasing arrangement so as to allow for electronic steering of the beams.
Basically, the beam width produced by the antenna defines the azimuth resolution that may be obtained; the beam width is proportional to the antenna length. For obtaining a high resolution target image this requires an antenna physically longer than can be carried by an airborne platform such as an aircraft or satellite. In a SAR mode, multiple radar returns are collected as the platform flies a predetermined course. The returns are combined and processed as though they came from a physically long antenna (synthetic aperture) yielding a finer resolution than is possible from a smaller antenna.
In one type of SAR, known as a spotlight SAR, as the platform flies its prescribed course, multiple transmissions are steered toward the same target area, from which multiple returns are collected and processed. The processing results in a target image which is of photographic quality.
The MTI mode is somewhat similar in that multiple transmissions are directed toward a target area, although less such transmissions are made compared to the SAR mode, to achieve a result. In the MTI mode Doppler returns are processed which depict moving targets within the target area.
In operation, multiple transmit pulses, one for each beam, are generated in a single interpulse period (IPP) and returns are detected within respective receive windows. For aircraft borne radars these receive windows may be in the same IPP as the transmit pulses, or for a spaceborne application the receive windows will be in a subsequent IPP as a function of the target range.
During such operation, it is imperative to avoid any collisions, where a collision is defined as the simultaneous time occurrence of any combination of transmit pulses and receive windows. Any such collisions during the dwell time that the beams are directed toward the target area will prevent obtaining of the desired target image.
Accordingly, the present invention has for an object a procedure for determining if a certain pulse repetition frequency (PRF), PRF change rate, and a certain alignment of transmit pulses and receive windows will allow sufficient dwell time to accomplish the collection of multiple images within the collection time of a single image.